This invention relates to an apparatus for measuring the velocity of ultrasonic sound in terms of V(z) characteristics, as well as an ultrasonic microscope using that apparatus. More particularly, this invention relates to an Ultrasonic microscope of a type the uses so-called "V(z) characteristics" to determine the velocity of a leaky elastic surface wave from a sample to be examined and which is adapted to achieve a higher resolution by reducing the amount of relative movement of the sample in the Z-direction (the axis parallel to the direction of the height of the microscope) to less than one period of the V(z) characteristic curve. The term "V(z) characteristics" as used herein means the periodic characteristics of the intensity of signal reception of a wave generated by the interference between two components of an ultrasonic beam that has been focused by an acoustic probe and that has been applied to the sample, i.e., the reflected wave from an area near the Z-axis and the reradiated leaky elastic surface wave from the sample. The reradiated leaky elastic surface wave is obtained by excitation with that component of the convergent ultrasonic beam which has been applied to the sample at an angle near the critical Rayleigh angle, and the V(z) characteristics are obtained by making a relative movement of the sample in a direction (-Z direction) in which the sample approaches the acoustic probe, with the origin being the focal point of the incident ultrasonic beam.
FIG. 7 is a partial enlarged cross section of the acoustic probe of an ultrasonic microscope. The acoustic probe indicated by 17 is fixed to the Z-scanning axis (height axis) of the measuring mechanism of the microscope, and usually consists of a transducer (thin piezoelectric film) and a lens. The transducer is driven intermittently at constant time intervals in response to burst signals from the measuring part, whereupon burst (ultrasonic) waves are emitted towards the surface of a sample 11 to be examined.
Among the emitted burst waves, the one that has travelled in the path G - A - B to fall on the surface of the sample 11 at the critical angle (.theta.Lsaw) is converted to a leaky elastic surface wave which, as it propagates from the incident point B through the surface of the sample 11, reradiates a wave at the critical Rayleigh angle, and the one that is reradiated at point C on the surface of the sample is emitted towards point D of the acoustic probe 17. The emitted wave follows the return path that is parallel to the path G in the probe in which the incident burst wave travelled, whereupon it returns to the other end of the piezoelectric device. Although only one sound wave is shown in FIG. 7 to propagate in the path G - A - B - C - D - H, the above explanation shall apply to all sound waves that propagate in the paths at the positions obtained by rotating the path G - A - B - C - D - H about the axis of the probe. The signal obtained by receiving those waves corresponds to the reradiated component of the aforementioned leaky elastic surface wave.
Besides this wave, a burst wave 24 that has been emitted from the probe 17 in a direction normal to the surface of the sample 11 travels in the path E - F - E to return to the piezoelectric device. This is the reflected wave from an area near the Z-axis also mentioned above. These two wave components have a phase difference that is based on the difference between their path lengths. As a result, the two wave components interfere with each other and the interference signal is received by the probe 17. Shown by W in FIG. 7 is a sound field-creating liquid medium.
If the distance of path E - F is varied, the phase difference between the two burst waves 23 and 24 due to the difference between their path lengths will change accordingly, producing interference variations in the received signal to create the so-called "V(z) characteristics" as shown in FIG. 8. The velocity of the leaky elastic surface wave travelling between points B and C through the surface of the sample can be calculated from the dip period .DELTA.z of the characteristics curve (the dip period is hereunder referred to simply as the "period"), as a result of which the surface of the sample can be analyzed. Details of this technique are already known by disclosure in many references including Weglein, Applied Physics Letters, 34(3), 179-181, Feb. 1, 1979.
As is clear from the V(z) characteristic curve shown in FIG. 8, in order to determine the period .DELTA.z in a correct way, it is necessary in practice to measure the intensity of interference wave over several periods in consideration of noise and precision of measurements. Under the circumstances, the sample 11 has to be moved relative to the acoustic probe 17 in the -Z direction by the amount necessary for the measurement. However, this movement has one serious problem: as one can understand from FIG. 7, the greater the amount of movement in the Z direction, the longer the distance of path B - C.
Increasing the distance of path B - C for the purpose of obtaining the correct value of the period .DELTA.z means taking data for the case where the diameter of the convergent beam increased. The velocity of the leaky elastic surface wave as calculated from the data corresponds to the increased length of path B - C. The resolution of data on such parameters as the nature and crystalline structure of the material of interest as obtained in a manner dependent upon the velocity of the leaky elastic surface wave and the resolution of the image that is depicted in a manner dependent upon the velocity of the same leaky elastic surface wave are limited by the length of path B - C, so the values of those resolutions cannot be made adequately high if the length of path B - C is not reduced.
In order to improve the resolution of the structural analysis of materials or that of ultrasonic images, one may either prevent the distance of path B - C from increasing or limit the distance of movement in the Z direction. However, the first approach is limited in terms of the critical angle and, hence, the scope of materials that can be analyzed is restricted, whereas the second approach which uses the V(z) characteristics to measure the period .DELTA.z has a theoretical difficulty in that the distance of movement must be slightly greater than the value necessary for determining one period of .DELTA.z, but cannot be made shorter than the corresponding length of path B - C.